The European Medicines Agency (EMA) head of vaccines Marco Cavaleri on Tuesday, said the regulator sees a potential link between the AstraZeneca COVID-19 vaccine and increased risk of blood clots, Italian newspaper Messaggero reported.

"In my opinion, we can say it now, it is clear there is a link with the vaccine. But we still do not know what causes this reaction," Cavaleri told the paper.

He said that "in the next few hours, we will say that there is a connection, but we still have to understand how this happens".

Persistent questions on whether rare but serious blood clots among those getting the AstraZeneca jab against Covid-19 are more frequent than in the general population have undermined confidence in the beleaguered vaccine.

After several countries suspended the use of the jab – including Italy – the EMA declared that the benefits outweigh the risks and it should remain in use.

But it said that a causal link between clots and the vaccine is possible and is expected to provide an updated assessment this week.

"We are trying to get a precise picture of what is happening, to define in detail this syndrome due to the vaccine," Cavaleri said.

He added: "Among the vaccinated, there are more cases of cerebral thrombosis ... among young people than we would expect."

The European Union has blocked shipments of 3.1 million doses of AstraZeneca's COVID-19 vaccine to Australia, and the country has little hope of getting the remaining 400,000 doses it has been pledged on time, reported Reuters, citing a government source on Tuesday.

(With input from AFP)